One of the legacy sites I still do a bit of maintenance on has a large
sealed thermal store with plate heat exchangers for DHW plus an underfloor
loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one
but nothing has been done about this in 4 years. Spare parts for the wood
boiler take about 3 days to source and fit so there is a need for a back
up.

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

"andrew" wrote in message
...
One of the legacy sites I still do a bit of maintenance on has a
large
sealed thermal store with plate heat exchangers for DHW plus an
underfloor
loop. It is normally solar powered with a wood burner for high
demand.

It has a 3ph delta wired immersion heater provided as back up but,
due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with
240V one
but nothing has been done about this in 4 years. Spare parts for the
wood
boiler take about 3 days to source and fit so there is a need for a
back
up.

I talked with the electrician and pointed out one could supply 240V
across
two of the immersion terminals and neutral on the third, which would
drive
~0.6 of the current and produce 1/3 of the heat of each element in
the
immersion. So the 9kW 3phase immersion would derate to 2 elements at
1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in
a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

If the six wires that are the two ends of each of the three heating
elements are accessable, then they just need wiring so that the
elements are in parallel to run off single phase 240v. I did precisely
this with a large three phase pottery kiln which was entirely
satisfactory (except for the eletricity bill!)

AWEM

On 5 Mar, 10:47, andrew wrote:
One of the legacy sites I still do a bit of maintenance on has a large
sealed thermal store with plate heat exchangers for DHW plus an underfloor
loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one
but nothing has been done about this in 4 years. Spare parts for the wood
boiler take about 3 days to source and fit so there is a need for a back
up.

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

I just did a fag packet calculation to check your power per element
and you are closely correct. The only thing I can see that you have
not mentioned is temperature control but for a three phase unit I
would expect it to be via a separate thermostat and contactor in the
supply. The coil voltage may be 400(415) and would require a coil
change.
Are the ends of the elements not accessible to split the delta at all?
If they are and providing you use high temperature connectors all
three elements could be used in parallel, otherwise your idea of
gaining the use of two with one leg of the delta shorted to either N
or L is sound.
Many "electricians" are sadly far from fully competent or hide behind
the "safety" mantra when in reality there is no risk once the work is
completed, tested and covers/enclosures replaced.

The question remains is 2 kW going to be enough for a meaningful
contribution or would it be a waste of time anyway?

Finally if it takes three days for spares for the wood burner to
arrive what is the cost of downtime and it may be worth keeping a set
of common parts on stock.

On Mar 5, 10:47*am, andrew wrote:
One of the legacy sites I still do a bit of maintenance on has a large
sealed thermal store with plate heat exchangers for DHW plus an underfloor
loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one
but nothing has been done about this in 4 years. Spare parts for the wood
boiler take about 3 days to source and fit so there is a need for a back
up.

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

Surely you could get 2/3 the power into it by connecting the 3
terminals to L,L,N or N,L,L.

Certainly nothing unsafe about it, as long as the single phase supply
is all upto it. Clearly not inappropriate either - but when people
react like that, any attempt at sense is futile.


NT

cynic wrote:

I can see that it would be in defensible as it is using equipment in a
way for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

I just did a fag packet calculation to check your power per element
and you are closely correct.

I just rounded 1 over root 3 by a bit.


The only thing I can see that you have
not mentioned is temperature control but for a three phase unit I
would expect it to be via a separate thermostat and contactor in the
supply. The coil voltage may be 400(415) and would require a coil
change.
Are the ends of the elements not accessible to split the delta at all?

It doesn't look like it, just 3 bolted terminals under the exterior cover. I
thought the temperature control would just be a bimetallic contact as in a
domestic immersion but if you're right then this is a worry. The system has
2 pressure relief valves (both set at 4 bar yet one is 3 storeys and 12m
below the other) and automatic water make up so little chance of damage.

There's 3tonne of water to heat so little chance of boiling.



The question remains is 2 kW going to be enough for a meaningful
contribution or would it be a waste of time anyway?

Maybe a waste of time but I imagine the occupants would be grateful of
something just to take the chill off. The worst heat demand is some
600kWhr/day and this is for DHW and space heating.

The stupid thing about the arrangement is that the top of the tank is not
reserved for DHW so the underfloor loop destroys stratification and lowers
the whole tank to one temperature when things go bad.

Finally if it takes three days for spares for the wood burner to
arrive what is the cost of downtime and it may be worth keeping a set
of common parts on stock.

The woodburner costs about 10 times as much as a condensing gas boiler. The
latest failure was a small stainless steel part in the fire box distorting
and then heat radiating onto some connector blocks in the grate area. The
silicon wires held up but the plastic connectors and plastic grate motor
cover melted. The new firebox is 500 quid, I have fitted one sent down by
an old friend from Scotland but it still took from the fault reported on
Friday to my deciding what needed doing on Sunday to being up and running
on Tuesday as I live 2 hours away and have work commitments. The new parts
will take 2 weeks but yes they should be kept in stock.

I replaced the connectors with porcelain ones and will consider fitting an
Aga fire stop valve to the control the feed auger.

Four years ago I asked for some sort of webserver to be used to flag up
faults as the 3 tonnes of water will keep the place going from hot to 25C
for 8hours at worst case. If they dedicated the top of the thermal store to
maintain DHW it would be much longer before the floor slabs were cold.

BTW when the load of each flat was calculated use of CRT televisions was
expected, presumably modern electronics waste less power as heat as the
system struggles.

AJH

On Mar 5, 11:45*am, John Rumm wrote:
NT wrote:
On Mar 5, 10:47 am, andrew wrote:
One of the legacy sites I still do a bit of maintenance on has a large
sealed thermal store with plate heat exchangers for DHW plus an underfloor
loop. It is normally solar powered with a wood burner for high demand.

It has a 3ph delta wired immersion heater provided as back up but, due to
cock ups in communication there is only a single phase supply.

Obvious solution is to pull the 3ph immersion out and replace with 240V one
but nothing has been done about this in 4 years. Spare parts for the wood
boiler take about 3 days to source and fit so there is a need for a back
up.

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

I can see that it would be in defensible as it is using equipment in a way
for which it isn't designed but unsafe? After all it's using 0.6 the
voltage and current it was designed for.

AJH

Surely you could get 2/3 the power into it by connecting the 3
terminals to L,L,N or N,L,L.

You would also be reducing the effective voltage, i.e. 240V L to N,
rather than 415V L1 to L2 etc.

yes...


Certainly nothing unsafe about it, as long as the single phase supply
is all upto it. Clearly not inappropriate either - but when people
react like that, any attempt at sense is futile.

Too true!

Don't overlook other space heat sources as well. Eg a hob can heat a
small or small-medium house temporarily.


NT

andrew wrote:

I talked with the electrician and pointed out one could supply 240V across
two of the immersion terminals and neutral on the third, which would drive
~0.6 of the current and produce 1/3 of the heat of each element in the
immersion. So the 9kW 3phase immersion would derate to 2 elements at 1kW
each, still better than nothing if the 240V supply is up to it. The
suggestion was immediately condemned as unsafe and inappropriate.

This hasn't gone any further, boiler parts haven't arrived and the system is
running on the loaned parts.

One other thing crossed my mind, there are 12 flats so I think there is
possibly 3ph coming to the site or it would present a bad unbalance to the
local grid, or is it common in London to have one phase supply several
houses?

At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

AJH

On 10/03/2010 21:15, andrew wrote:

At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

Not really, no. By convention, the solution to your problem is to go to
the DNO and ask (pay) for a bigger supply to meet your demand.

Implementing automatic load-shedding on this scale would involve a
control panel manufacturer building you a bespoke solution, which would
consist of a set of current transformers and an energy analyser to
monitor current. The analyser would need to have presetable alarm
contacts which would operate a relay when the preset current level was
reached in any phase. The relay (along with some front panel override
controls, etc.) would operate a contactor which cut out the supply to
the heaters.

I would expect a commercial price in the thousands to implement this.

If you had a building management system (BMS) which was monitoring
demand, you could leverage this system to load-shed.

On Mar 16, 10:50*am, Dave Osborne wrote:
On 10/03/2010 21:15, andrew wrote:

At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

Not really, no. By convention, the solution to your problem is to go to
the DNO and ask (pay) for a bigger supply to meet your demand.

Implementing automatic load-shedding on this scale would involve a
control panel manufacturer building you a bespoke solution, which would
consist of a set of current transformers and an energy analyser to
monitor current. The analyser would need to have presetable alarm
contacts which would operate a relay when the preset current level was
reached in any phase. The relay (along with some front panel override
controls, etc.) would operate a contactor which cut out the supply to
the heaters.

I would expect a commercial price in the thousands to implement this.

If you had a building management system (BMS) which was monitoring
demand, you could leverage this system to load-shed.


Or you could get some small scale shop to make one up for several
hundred. The technology's simple. It can be a lot simpler than the
above.

3 relays with their coils rewound so they switch on at around 50A
current (relay winding across a shunt, you dont want all 50A thru the
relay). You have one of these in each phase feed.
The switch contacts run in series such that you only get power out if
all 3 phases are loaded at under say 50A
And this power line operates a 3 phase relay, or 3 single ones, to
switch the heater feed on and off.

You can adjust switching sensitivity by altering the shunt.
Need to do calcs, this scheme may need tweaking.


NT

On 16/03/2010 15:59, NT wrote:
On Mar 16, 10:50 am, Dave wrote:
On 10/03/2010 21:15, andrew wrote:

At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

Not really, no. By convention, the solution to your problem is to go to
the DNO and ask (pay) for a bigger supply to meet your demand.

Implementing automatic load-shedding on this scale would involve a
control panel manufacturer building you a bespoke solution, which would
consist of a set of current transformers and an energy analyser to
monitor current. The analyser would need to have presetable alarm
contacts which would operate a relay when the preset current level was
reached in any phase. The relay (along with some front panel override
controls, etc.) would operate a contactor which cut out the supply to
the heaters.

I would expect a commercial price in the thousands to implement this.

If you had a building management system (BMS) which was monitoring
demand, you could leverage this system to load-shed.


Or you could get some small scale shop to make one up for several
hundred. The technology's simple. It can be a lot simpler than the
above.

There's nothing complicated about the above.

3 relays with their coils rewound

OK, where are you going to get relays with their coils rewound?

so they switch on at around 50A
current (relay winding across a shunt, you dont want all 50A thru the
relay). You have one of these in each phase feed.
The switch contacts run in series such that you only get power out if
all 3 phases are loaded at under say 50A
And this power line operates a 3 phase relay, or 3 single ones, to
switch the heater feed on and off.

In what way would "3 single ones" be acceptable?


You can adjust switching sensitivity by altering the shunt.
Need to do calcs, this scheme may need tweaking.


NT

An interesting solution. However, in broad terms, your solution is no
simpler than mine. It has comparable design, construction, installation
and commissioning time to my solution and may well (by your own
admission) require more tinker time. It would likely save money on
parts, I agree. However, if you farmed it out to a contractor, the
labour charge would far outweigh the parts costs.

Note that whatever solution you employ, you would need to install a
separate consumer unit for the control and the heater, the problem being
that you need to monitor demand excluding the heater load, otherwise the
system will flap.

On Mar 16, 5:46 pm, Dave Osborne wrote:
On 16/03/2010 15:59, NT wrote:

On Mar 16, 10:50 am, Dave wrote:
On 10/03/2010 21:15, andrew wrote:


At work we have a 100A 3ph supply with several buildings being on separate
phases and only the workshop with 3ph. If the flats are the same and given
that there may not be enough peak capacity to supply the immersion and all
the flats at once; is there a device that would switch off the immersion if
the total load on the 3ph supply exceeded 80A and then only switch it back
in when it fell below 50A in any one phase?

Not really, no. By convention, the solution to your problem is to go to
the DNO and ask (pay) for a bigger supply to meet your demand.

Implementing automatic load-shedding on this scale would involve a
control panel manufacturer building you a bespoke solution, which would
consist of a set of current transformers and an energy analyser to
monitor current. The analyser would need to have presetable alarm
contacts which would operate a relay when the preset current level was
reached in any phase. The relay (along with some front panel override
controls, etc.) would operate a contactor which cut out the supply to
the heaters.

I would expect a commercial price in the thousands to implement this.

If you had a building management system (BMS) which was monitoring
demand, you could leverage this system to load-shed.

Or you could get some small scale shop to make one up for several
hundred. The technology's simple. It can be a lot simpler than the
above.

There's nothing complicated about the above.

3 relays with their coils rewound

OK, where are you going to get relays with their coils rewound?

any electrical wholesaler for the contactors. Enamelled coper wire
comes from any electronic parts seller, eg rapidonline


so they switch on at around 50A

current (relay winding across a shunt, you dont want all 50A thru the
relay). You have one of these in each phase feed.
The switch contacts run in series such that you only get power out if
all 3 phases are loaded at under say 50A
And this power line operates a 3 phase relay, or 3 single ones, to
switch the heater feed on and off.

In what way would "3 single ones" be acceptable?

in all ways afaik. Could you be more specific?


You can adjust switching sensitivity by altering the shunt.
Need to do calcs, this scheme may need tweaking.

NT

An interesting solution. However, in broad terms, your solution is no
simpler than mine.

4 or 6 relays versus:

control panel manufacturer building you a bespoke solution, which would
consist of a set of current transformers and an energy analyser to
monitor current. The analyser would need to have presetable alarm
contacts which would operate a relay when the preset current level was
reached in any phase. The relay (along with some front panel override
controls, etc.) would operate a contactor which cut out the supply to
the heaters.


It has comparable design, construction, installation
and commissioning time to my solution and may well (by your own
admission) require more tinker time.

It shouldnt require any. Relays typically pull in in the region of
half rated voltage/current, and we dont need much accuracy on this
system.


It would likely save money on
parts, I agree. However, if you farmed it out to a contractor, the
labour charge would far outweigh the parts costs.

yup. With just 4-6 relays its a simple job.


Note that whatever solution you employ, you would need to install a
separate consumer unit for the control and the heater, the problem being
that you need to monitor demand excluding the heater load, otherwise the
system will flap.

good point. Or we could monitor total current from before the CU, then
add a 2nd winding (connected the opposite way) on the sense relay,
monitoring the water heater current to cancel its effect out. Either
works.

Be better really to not connect the 3 sense relays to each other, then
the control for each phase is independant, and the heater is also
available more of the time at lower power. The main advantage of this
is better reliability, any component failure doesnt affect 2 of the 3
heating elements.


NT

NT wrote:

On Mar 16, 5:46 pm, Dave Osborne wrote:
On 16/03/2010 15:59, NT wrote:

On Mar 16, 10:50 am, Dave wrote:
On 10/03/2010 21:15, andrew wrote:

Thanks for the input you two it's all interesting and pertinent.

AJH

On Mar 16, 7:46*pm, andrew wrote:
NT wrote:
On Mar 16, 5:46 pm, Dave Osborne wrote:
On 16/03/2010 15:59, NT wrote:

On Mar 16, 10:50 am, Dave *wrote:
On 10/03/2010 21:15, andrew wrote:

Thanks for the input you two it's all interesting and pertinent.

AJH

Here on the other side of the Atlantic I have to agree with a previous
poster to this thread who wrote ..............

"Many "electricians" are sadly far from fully competent or hide
behind
the "safety" mantra when in reality there is no risk once the work is
completed, tested and covers/enclosures replaced."

Often found during a 40 year career in telecommunications, which often
involved power at many voltages and frequencies, that many
electricians knew much about 'wiring methods', how many inches of
slack, what gauge of wires and how many would fit through something
(e.g. conduit) etc. and by 'Rule of thumb' what size of wire or
circuit breaker to use etc. Many/most? had little idea of electrical
theory, or basic AC/DC principles. They definitely knew more than I
did when it came to 'Wiring something'!

But; having been trained in electrical theory, at many frequencies,
from audio to microwaves and how to repair and maintain electronic
equipment we were a little astounded that there was so little
knowledge of 3 phase, or even two phase and the relationships of
voltages. And mention mention 400 hertz AC or DC at say 500 amps, or
some old time wood/paper pulp mill 25 cycle motors or equipment some
of them and it didn't have any impact!

Now many years later my son who is fully trained Instrumentation
Technician (Off shore oil/gas with megawatts of power and very high
pressures) who has also done some teaching, is finding the same. Many
electricians 'may' know rudimentary volts and amps but not much
theory! Something missing in the training/technology courses?

Cos we don't see that deficiency here where there are some pretty
knowledgeable and practical people ready to advise.

Agree don't see why those three heating elements (control switching
aside) can't be paralleled and run at the reduced power. While may
need appreciation of whether the unit comes factory wired for 415 volt
delta or wye/star, should be do-able safely?

I do recall rigging up some sort of European/UK sourced small potters-
craft-maker's kiln (single phase but requiring rewiring for our two
hot 120 volt with 240 between them, as used for out hot water heaters
and cooking stoves) in some way years ago and apparently it worked for
years, safely, with no problems.